Problem with unique and nullable properties

Maybe I'm missing something... but I can add the record (5, null) two times!
I thought the unique constraint would prevent it or
maybe there is a basic explanation for this way, can anybody tell me why?

As Burt mentioned if you allow nulls you can not. But if your
semantics allow it you could probably make one value as a mark value
-1 or 0 and use that instead of null. Then you can have non-null field
and DB will start enforcing the constraint.

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